The present invention relates to a method of joining Ni-base heat-resisting alloys which method is suitable for fabricating such parts being made of Ni-base heat-resisting alloys and having complex shapes as those for a heat engine, e.g., high temperature turbine blades.
Usually, Ni-base heat-resisting alloys have been employed as a material for high temperature gas turbine blades, the inner structure of which has a cooling system comprising complicated cooling paths in order to raise the operating temperature.
As the typical structures, there may be mentioned (A) a return-flow type precision cast blade and (B) a wafer blade in which ten and several or more pieces of wafer are joined in layers.
In cases where a high temperature gas turbine equipped with such complicated cooling paths is to be fabricated, the diffusion-joining method is an important technology. Namely, in case of (A), as shown cross-sectionally in FIG. 1, after each half of the blade which has been divided into two parts is precision cast and then these parts are combined together by placing an insert filler metal at the curved surfaces to be joined, the parts are integrated through the diffusion-joining process.
In order to join such broad surfaces with curvature, thicker filler metal is desirable from the standpoint of the dimensional tolerance required.
Further, in case of (B), thinner filler metal is desirable because there are many surfaces to be joined and besides high dimensional accuracy of each plane is required.
Although a high-temperature isobaric compression technique may be employed as another diffusion-joining method to join curved surfaces, this technique requires a special large-sized press as well as a capsulating and a masking technique, and therefore it is intricate and expensive. Consequently, its practicality is problematic.
For the manufacture of blades (A) and (B), a liquid phase diffusion-joining method is normally used to assure the reliability of the joining. That is, the filler metals such as Ni-P, Ni-Si, Ni-Cr-B and so forth have been heretofore used to join Ni-base heat-resisting alloys. In short, such a filler metal, which has been prepared by adding to nickel a melting point-lowering element such as boron, phosphorus or silicon, is melted at a temperature lower, by several tens of degrees, than the melting point of the Ni-base heat-resisting alloy material used as the body material to momentarily wet and braze the Ni-base heat-resisting alloy material, and then a heat treatment is carried out for a long period of time so that the boron, phosphorus or silicon may diffuse, whereby an isothermic solidification of the filler metal as well as the body material is caused, and a strongly joining state is resultingly obtained (Japanese Provisional Patent Publication No. 13060/1974).
However, while this conventional method gives an excellent reliability of the joining, there are, on the other hand, two problems as follows.
First, there is a possibility that B and P, which are melting temperature lowering elements, are contained in the joining portion (joint) to impair its corrosion resistance at higher temperatures and its ductility at higher temperatures. This fact shows that the conventional method is not necessarily sufficient as a method of joining structural parts made of the same Ni-base heat-resisting alloy which is used under severe operating conditions at higher temperature.
The second point is the problem that the method of supplying the filler metal is restricted. The filler metal contains a melting temperature lowering element so that its workability is extremely impaired. Therefore, there has been employed a sheet made of powder using an organic binder, or an amorphous ribbon prepared by the rapid-cooling method.
However, in case of the former, the handling of the sheet is unstable, and there are problems of the contamination by the residue of the binder and of the dimensional shrinkage at the time of melting the sheet. In the latter case, a thickness of only several ten microns can be attained. Thus, the degree of freedom in thickness of the filler metal ranging between thin type and thick type necessary for preparing turbine blades of (A) and (B), can not be obtained.
Further, a plating method and a vapor deposition method have been proposed for the purpose (see Japanese Patent Publication No. 29984/1975). In the former method, however, the composition of the filler is limited to Ni-P and the like, and there is caused a problem of the surface contamination inherent in the wet plating technique. In the latter method, there are problems of the fluctuation and ununiformity of the composition in the deposited material. Thus, there has not been developed any method which has practically high degree of freedom in supplying the filler metal.